It is known in the art that "bump steer" is a problem in that toe-in changes during vertical displacement of the wheel are undesirable and cause a change in the dynamic behavior of the vehicle. While particularly important for racing vehicles, it is also undesirable in conventional automobiles. On vehicles equipped with independent suspension, it is common practice to define side view swing arm geometry by appropriately arranging the control arms of each wheel in a way that causes their axes to converge at some instantaneous point(s) yielding a desired percentage of anti-dive/lift by inducing vector forces to counter weight transfer arising out of acceleration and braking. In normal practice, uprights attach to the outboard end of control arms via spherical joints, while the steering arms are rigidly attached to the uprights. Strut type suspension systems utilize only a lower control linkage, as a telescoping strut integrated with the upright replaces the upper control linkage used in the SLA (short-long-arm) layout.
In a vehicle's front wheel steering system and/or the rear wheel toe-control system, it is common practice to have the steering arms rigidly attached to the uprights carrying the road wheels, for transferring directional input from the operator, or from said toe-control link.
In side elevation, the angle between a vertical and a straight line passing through the upright's spherical bearings is commonly referred to as castor angle. Since the rotational planes of associated control linkages intersect one another, the castor angle is subjected to incremental changes as the wheel drops and rises.
Since the tie-rod's inboard pivot is attached to an actuator, usually in the form of a rack or a link rod, which is firmly anchored to the chassis in both vertical and fore-and-aft planes, or as is the case in a rear suspension, the toe-control link's inboard end is attached to a fixed point on the chassis, a conventional steering arm has a drawback as any incremental change of the castor angle produces a corresponding change in the steering arm's slope, thereby inducing twist in the imaginary surface formed by the steering arm, the tie-rod and the control linkage causing toe-in changes in the wheel as it rises and drops.
It is of course necessary that the projected length of the steering arm, as measured on a reference plane, be held in check for minimizing undesirable toe-in changes during the suspension's compression and extension strokes in the presence of anti-dive/anti-squat geometry. To minimize changes in the instantaneous castor angle, it is customary to position the control linkage pivot axes at some convenient angle to the vehicle's longitudinal centre line, as observed in plan view. Arising out of this arrangement, the control linkage tips rotate on elliptical paths, as viewed in side elevation, hence the changes may only be minimized, but not eliminated.